What I am asking for is if there is any theory related to this real series $f(s)=\sum_{n=1}^\infty e^{-n^s}$ and $s\ge 1$.
As far as I know, if $s = 1$, it's a simple geometric series, and if $s = 2$, the series can be converted to one case of Riemann Theta Function. So what if $s$ is generalized to any real number larger than or equal to 1?